Theophylline and theobromine are other alkaloids, which are also present in some drinks. These compounds are very similar in chemical structure to caffeine.

Theophylline is found in small amounts in tea but has a stronger effect on the heart and respiratory system than caffeine.

Theobromine is found in cocoa products and small amounts in tea, and is mainly responsible for the stimulating effects in cocoa products.

Caffeine tablets such as NoDoz can be obtained without a prescription and are used to increase alertness.

Caffeine has been used as a weight-loss aid in the past because of its ability to improve metabolism. But it has not been used for this purpose for many years because there is no evidence that it helps people to lose weight.

Caffeine is also used in combination with painkillers such as aspirin to provide headache relief, but there is little evidence for its use for this purpose.

How much is too much?

A high intake of caffeine at once can cause caffeine-induced mental disorder. The symptoms include restlessness, nervousness, excitement, insomnia, stomach upset, muscle twitching, incoherent speech, heart palpitations and excessive alertness.

In most cases an overdose of caffeine will not kill you. However, it can be fatal. A lethal dose is estimated at 10 grams. However, a toxic dose will vary from person to person.

Even at much lower doses, too much caffeine can have unwanted side effects.

If you are drinking more than 10 cups of coffee a day you should seriously consider reducing your intake.

Moderate caffeine consumption is considered to be about 300 mg, which is equivalent to 3 cups of coffee, and varies depending on the strength of the coffee.

The American Medical Association has reported that moderate coffee drinkers should not be concerned about their health provided they also lead a healthy lifestyle.

There is no evidence linking caffeine consumption to cancer or heart disease. Some recent research has suggested caffeine may increase homocysteine levels (a risk factor for heart disease). The actual clinical implications of this are unknown at present.

Coffee consumption can cause a rise in blood pressure for a few hours in people sensitive to the effects of caffeine, but this is less than what is normally experienced from climbing stairs. However, patients with high blood pressure should talk to their doctor about caffeine consumption.

Large caffeine consumption in the elderly has been linked to an increased risk of hip fracture and loss of bone density.

It may also aggravate the effects of irritable bowel syndrome.

High coffee consumption has also been linked to problems in pregnancy. Caffeine has caused birth defects in rats when consumed at a comparable level of 70 cups a day for humans. There is no evidence that caffeine is linked to birth defects in human babies.

The strongest evidence is that caffeine may reduce a baby's birth weight. However, this effect is confined to mothers who also smoke during pregnancy, and even then the evidence is borderline.

Caffeine has also been shown to reduce sperm movement.

Withdrawal effects:

Caffeine withdrawal can produce several side effects. These include:

Headaches

Irritability

Nervousness

Restlessness

Tiredness

Hints for cutting caffeine intake:

Reduce the amount of caffeine slowly to help avoid withdrawal symptoms. Cutting down at a rate of about 1/2 cup a day seems to avoid most side effects.

It may be helpful to keep a guide of how much caffeine you are consuming (including soft drinks) per week and begin cutting down slowly.

Some people prefer to suddenly stop drinking caffeine by going "cold turkey", however, the withdrawal effects can be quite severe with this method.

People experiencing withdrawal symptoms may find it helpful to drink a Coca-Cola.

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